As the world population is increasing, the number of vehicles is expected to increase from 1.2 billion to 2 billion by 2035. This results in the production of nearly 1.5 billion tires per year, which inevitably turn into waste. These discarded tires pose serious environmental and health issues such as: 1) piles of the waste tires generate vermin and insect infestation, which pose a severe health issue; 2) if piles of waste tires catch fire, it is hard to extinguish and moreover burning tires generate hazardous gases, heavy metals and oil which severely contaminate soil, environment and groundwater; and 3) waste tires require large landfills and increasing number of discarded tires may lead to landfill maintenance issues.
Owing to their cross-linked structure and presence of various additives, waste tires do not decompose easily and resist degradation in most of the chemically and physically harsh conditions. As a result, they are generally disposed of and accumulate in landfills, which is an unsustainable solution. The best solution economically and ecologically would be to recycle waste tires and use them as a raw material for value-added products. A typical tire contains natural rubber and polybutadiene, styrene butadiene rubber, butyl rubber and fractional amounts of organic and inorganic fillers/additives. Carbon black is the major constituent of the tire (˜30-35% by weight), which provides an avenue to recover this carbon for energy related applications. A method of recovering carbon black from waste tires is disclosed in U.S. patent application Ser. No. 13/945,239, “Pyrolytic Carbon Black Composite and Method of Making the Same,” filed Jul. 18, 2013, the disclosure of which is incorporated fully by reference.
In spite of the significant efforts to develop carbon-based materials for supercapacitors, such as reduced graphene oxide (rGO), multiwall carbon nanotubes (MWCNT), carbide derived carbons (CDC), and onion-like carbons (OLC), activated carbon (AC) still remains the material of choice for commercial supercapacitors due to its moderate cost, high specific surface area (SSA), good conductivity, high electrochemical performance and compatibility with other materials, for example, conducting polymers. Therefore, inexpensive synthesis of AC with developed porosity using biomass, waste products or other low cost and environmentally benign precursors has been explored for the development of next generation supercapacitor technology.
Towards this goal, synthesis of the AC from biomass and waste products has gained significant interest recently. For example, Wang et al produced high surface area (up to 2287 m2/g) nanosheets from hemp bast fibers, which demonstrated high charge storage capacity of 106 F/g at 0° C., at current density of 10 A/g in ionic liquids. Elmira et al. derived low surface area carbon from banana peels. When tested as a lithium-ion battery anode, the recovered carbon showed high gravimetric capacity of 1090 mAh/g at 50 mA/g. Jian et al. reported the recovery of carbon fibers from disposable bamboo chopsticks, which have abundant natural cellulose fibers. These carbon fibers in combination with manganese oxide yielded a capacity of 710 mAh/g for 300 cycles, when tested as lithium-ion battery anode. Li-Feng et al. pyrolyzed nitrogen doped bacterial cellulose to synthesize nitrogen rich carbon, which demonstrated 195 F/g capacitance at 1 A/g. Mandakini et al. pyrolyzed dead leaves to recover carbon (SSA 1230 m2/g), which yielded 88 F/g capacitance in organic electrolytes. Ping et al. used plant derived nitrogen rich carbon as a catalyst for the oxygen reduction reaction. Wenjing et al. derived heteroatom doped porous carbon by carbonizing human hairs, which exhibited a capacitance of 340 F/g at a current density of 1 A/g. Jia et al. synthesized carbon from the peanut shell for sodium-ion capacitor with a high rate capability of 72% after 10,000 cycles. Other waste materials used as carbon source include agricultural waste, rice husk, newspapers, wood and coffee.
A major limitation of AC materials is their poor energy density. To enhance the energy density, various approaches have been explored such as use of the pseudocapacitive materials (e.g., metal oxides or organic redox polymers) on conductive substrates, use of asymmetric configuration with expanded voltage windows, among others. Organic redox polymers, such as polyaniline (PANI), are promising pseudocapacitive materials because they are highly conductive, offer enhanced capacitance, and are environmental friendly and low cost materials. However, its large volume expansion and shrinkage during the doping/dedoping process leads to structural breakdown and limited lifetime performances. Most organic conducting redox-active polymers based electrodes lose more than 50% of their capacitance after 1000 cycles. One common strategy to improve the lifetime of organic-systems is to create composite materials based on strong conducting substrates such as carbon. Various attempts have been made to synthesize composites of PANI with various conductive substrates such as CNTs, rGO, metal oxides, GO (non-conductive), CDC, OLC, to benefit from the synergistic effects of both polymers and carbon materials. However, some of them rely on tedious synthesis methods and use toxic/explosive chemicals during the synthesis process.